cl-gdata is a library that provides a Common Lisp API to various Google Services. The library is still under active development, but it is already somewhat usable.
Currently, the following API's are at least partially implemented:
Document List API
Google Spreadsheets
Picasa
Contacts
Google Code Issue Tracker
Google Calendar
The only authentication method that is supported is ClientLogin, although the authentication is modular, so it shouldn't be difficult to implement support for other methods such as OAuth.
The source code can be found at Google Code project hosting: http://code.google.com/p/cl-gdata/